function fn = getFilters

 fn(:,:,1) = [0 0 0; -1 1 0; 0 0 0];
 fn(:,:,2) = [0 0 0; -1 1 0; 0 0 0]';
% 
%  fn(:,:,1) = [0 0 0; 1 0 -1; 0 0 0];
%  fn(:,:,2) = [0 0 0; 1 0 -1; 0 0 0]';
